In the continuous galvanizing of steel sheet, it is known that either a bath consisting of zinc and 0.10%-&lt;0.15% by weight of aluminium or a bath consisting of zinc and &gt;0.15-0.20% by weight of aluminium is most often used. The first type of bath is employed when the zinc-coated sheet, after the coating has been wiped, is subjected to a diffusion heat treatment so as to transform the zinc coating into a coating of an iron-zinc alloy, this commonly being called "galvannealing". The second type of bath is used to produce conventional galvanized product, that is to say sheet coated with a thin layer of zinc. If the bath contains less than 0.12% of aluminium, a whole range of iron-zinc compounds, as is described in the zinc-iron phase diagram, is formed at the interface between the iron and the zinc, which compounds are to be avoided in the production of conventional galvanized product. In order to prevent any nucleation of the .delta. phase, the aluminium must, in fact, be greater than 0.15%. This is the reason why the second type of bath has an aluminium content of more than 0.15%. For an aluminium content of approximately 0.15%, a very thin layer of Fe.sub.2 Al.sub.5 alloy, which will act as a barrier to any subsequent diffusion, is formed at the surface of the steel. This is the reason why the first type of bath has an aluminium content of less than 0.15%. However, this first type of bath requires the presence of at least approximately 0.10% of aluminium in order to slow down the reaction between the iron and the zinc while the sheet is passing through the bath; otherwise, this reaction would give rise to an excessive growth of the coating in the bath. Although slowed down by the aluminium, the reaction between the iron and the zinc nevertheless causes the formation of iron-zinc dross which accumulates at the bottom of the bath and which is consequently called bottom dross. This bottom dross stops being formed as soon as the aluminium content exceeds 0.15%; it therefore does not form in the second type of bath. In the second type of bath, part of the aluminium reacts with the iron of the sheet in order to form Fe.sub.2 Al.sub.5 compounds commonly called floating dross. It is also known that, in the continuous galvanizing world, three categories of galvanizers exist: those which just make "galvannealed" sheet, those which just make conventional galvanized sheet and those which make alternately, and without interruption, both types of sheets on one and the same galvanizing line. The latter use the first type of bath to make "galvannealed" sheet and the second type of bath to make conventional galvanized sheet, and they raise the aluminium content of the bath in order to pass from the first type of bath to the second: they therefore apply a process as defined hereinabove. This known process has the drawback that, because of the increase in the aluminium content of the bath when passing from the first to the second type of bath, the iron-zinc bottom dross existing in the bath at this moment will be progressively converted into iron-aluminium floating dross, and will rise and create defects on the sheet passing through the bath; there is therefore a risk of producing a substantial quantity of sheet of lower quality every time one passes from the production of "galvannealed" sheet to the production of conventional galvanized sheet.